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Re: [PATCH 2/2] Add gdb::string_view
On 03/18/2018 12:10 AM, Simon Marchi wrote:
> On 2018-03-17 19:49, Simon Marchi wrote:
>> We had a few times the need for a data structure that does essentially
>> what C++17's std::string_view does, which is to give an std::string-like
>> interface (only the read-only operations) to an arbitrary character
>> buffer.
Great, I've been wanting to do something like this for a while.
>>
>> I first copied the string_view file from today's gcc master
>> (b427286632d7) and adapted it (I don't think there should be any legal
>> issues since the copyright should already belong to the FSF):
>>
>> - I removed things related to wstring_view, u16string_view and
>> u32string_view (I don't think we need them, but we can always add them
>> later).
>> - I removed usages of _GLIBCXX_BEGIN_NAMESPACE_VERSION and
>> _GLIBCXX_END_NAMESPACE_VERSION.
>> - I put the code in the gdb namespace. I had to add a few "std::" in front of
>> std type usages.
>> - I added a constructor that builds a string_view from an std::string,
>> so that we can pass strings to string_view parameters seamlessly.
>> Normally, that's handled by "operator __sv_type" in the std::string
>> declaration, but it only exists when building with c++17.
>> - When building with >= c++17, gdb::string_view is an alias of
>> std::string_view.
>>
>> The result is close enough to the original file that if we ever need to
>> update it, it should be easy enough to compare it with the new version
>> in a diff editor and merge the new changes in.
>
> Hmm, when building with older g++ (such as the aarch64 builders on the buildbot,
> which have g++ 4.8), it trips on:
>
> using __idt = std::common_type_t<_Tp>;
>
> It looks like that release of g++ didn't have std::common_type_t. I guess it
> would be possible to avoid using it, and change these:
It has std::common_type though. C++14 std::foo_t types are usually just a
helper/convenience alias template, like:
/// Alias template for common_type
template<typename... _Tp>
using common_type_t = typename common_type<_Tp...>::type;
So it sounds like we can just use the C++11 / ::type form directly, or
add gdb::common_type_t somewhere, like common/traits.h or to our
copy of string_view.
>
> operator==(basic_string_view<_CharT, _Traits> __x,
> __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
>
> for
>
> operator==(basic_string_view<_CharT, _Traits> __x,
> basic_string_view<_CharT, _Traits> __y) noexcept
>
> but I am not aware of what consequences it would have.
Would it be possible to import some of the libstdc++'s relevant
testscases into our unit tests framework, like was done for
gdb::optional [1]?
[1] https://sourceware.org/ml/gdb-patches/2017-04/msg00239.html
Thanks,
Pedro Alves